The development of modern synthetic rubber materials have made possible the manufacture of a wide variety of elastomeric articles having varying properties of strength and chemical resistance. Among these articles are gloves designed for either industrial or medical uses. As safety accessories, industrial and medical gloves protect a user from environmental hazards such as chemicals or pathogens. In particular, medical gloves contribute to sanitary hospital conditions by limiting exposure of patients to potentially infectious matter, and serve to protect health professionals from disease transmission through contact with body fluids.
Relatively thin and flexible industrial or medical gloves have traditionally been made of natural rubber latex in a dipping process. The donning surface (i.e., the interior) of these gloves is conventionally coated with corn starch, talcum, or lypcopodium powder to lubricate the gloves, making them easier to don. In recent years, powder-free work gloves and medical gloves have largely replaced powdered gloves because of changing needs and perceptions of glove consumers. For example, cornstarch or other powders can impede healing if it gets into tissue (as during surgery). Similarly, powders are unsuitable for clean rooms such as those used in the manufacture of semiconductors and electronics.
Glove consumers have been moving away from natural rubber gloves due, in part, to an increasing rate of significant allergic reactions to proteins in natural rubber latex among health professionals as well as the general population. The industry has increasingly moved to latex emulsions based on synthetic rubber materials. While hospitals, laboratories, or other work environments that use rubber gloves often want to go “latex free” to better protect their workers, the higher cost of non-latex products, such as nitrile rubber, often limits their ability to make the change. For example, nitrile rubber gloves may cost two or more times the price of the natural rubber latex or vinyl-based counterparts. This fact has often caused purchasers in cost-sensitive environments, such as many hospitals, either to switch to less expensive polyvinyl chloride gloves or prevented them from switching to the synthetic materials.
In addition to being more expensive, nitrile-butadiene rubber medical exam gloves are typically stiffer and are perceived as much less comfortable to wear in comparison to similar gloves made from natural rubber latex materials. For instance, natural rubber latex (NRL) medical exam gloves typically require a stress of about 2.5 MPa (362.5 psi) to stretch to an elongation of about 300 percent of its original dimensions. This often is referred to as the glove's 300 percent modulus. Nitrile rubber medical exam gloves, on the other hand, typically require more than twice that amount of stress (˜6-8 MPa, ˜870-1160 psi) to achieve the same 300 percent elongation. While polyvinyl chloride medical exam gloves can be inexpensive, polyvinyl chloride medical exam gloves are typically considered a lower performance choice. That is, polyvinyl chloride medical exam gloves are typically stiffer and less elastic than even the conventional thicker nitrile rubber medical exam gloves.
Several previous approaches to softening nitrile rubber medical exam gloves involved strongly limiting or completely omitting zinc oxide and other materials capable of ionically crosslinking carboxylated nitrile rubber, such as those described in U.S. Pat. Nos. 6,031,042 and 6,451,893. In addition to not yielding force-strain properties similar to those of comparable natural rubber medical exam gloves, this method requires higher curing temperatures, a need for higher levels of other chemicals that may cause skin irritation, or may lead to processing difficulties such as thickening of the nitrile latex before dipping.
Other approaches to making a nitrile-butadiene rubber medical exam glove more comfortable, such as those described in U.S. Pat. Nos. 5,014,362 and 6,566,435, have relied on stress relaxation over time and require constantly applied levels of strain to cause that relaxation or softening. Such determination measures are difficult to maintain and are considered impractical or economically unfeasible.
While it might seem that a practical solution to the expense of conventional nitrile rubber medical exam gloves would be to make nitrile rubber medical exam gloves thinner than conventional nitrile rubber medical exam gloves (e.g., about 0.11 to about 0.20 mm in thickness at the palm region of the glove as measured generally in accordance with ASTM D3767, procedure A), there are significant problems associated with making nitrile rubber medical exam gloves that are thinner than conventional nitrile rubber medical exam gloves. A primary problem is pinhole formation which is sometimes referred to as “pinholes” or “pinhole defects. The lack of a thin nitrile rubber medical exam glove in the marketplace actually highlights the difficulties of economically and effectively solving these problems.
In the field of dipped rubber articles and in the field of breathable, stretched micro-porous films, a conventional solution is to pinhole defects is to utilize multiple thin layers of material. For example, PCT International Publication WO 1999/030904 A1 proposes in the manufacture of thin breathable films such as stretched micro-porous films that the use of a multilayer film greatly reduces or eliminates the probability of an imperfection (i.e., a pinhole) in any one area of one layer of the film aligning with an imperfection (i.e., a pinhole) in the other layer of the film, thereby substantially increasing the probability that the material produced will meet ASTM barrier test requirements. However, forming multilayer thin films adds complexity and expense to the manufacturing process and defeats the cost advantages provided by making an article thinner.
Similarly, U.S. Patent Application Publication No. 2008/0138723 A1 discloses nitrile rubber latex formulations and a process to make a multi-layered elastic glove in which the thickness of the multi-layered glove is between 0.01 mm and 0.3 mm. Such a multiple thin-layer dipping process to form thin multilayered gloves adds significant complexity and expense to the manufacturing process and defeats the cost advantages provided by making an article thinner. Importantly, the lack of a thin multi-layer nitrile rubber medical exam glove in the marketplace actually highlights the difficulties of economically and effectively solving these problems.
Although comparatively inexpensive, polyvinyl chloride medical exam gloves have a number of shortcomings. The shortcomings of polyvinyl chloride medical exam gloves include: being relatively inelastic; having relatively low tensile strength; having relatively greater amounts of pinhole defects; and leaching certain toxic components. These shortcomings can result in less comfort for the wearer, a weaker glove with higher permeability or poorer barrier protection against some common chemicals, and harm to the user and/or environment. Polyvinyl chloride medical exam gloves typically have a leakage percentage rate of from about 16 percent to about 44 percent when subjected to conventionally accepted leak testing. Conventional nitrile rubber medical exam gloves exhibit leakage percentage rates of less than 7 percent, typically less than about 5 percent or even lower (e.g., less than 2 percent). Reports of this comparative testing may be found at, for example, Kerr L. N., Chaput M. P., Cash L. C., et al., 2004 September. Assessment of the Durability of Medical Examination Gloves, Journal of Occupational and Environmental Hygiene 1: 607-612; Kerr L. N., Boivin W. S., Chaput M. P., et al., 2002 September. The Effect of Simulated Clinical Use on Vinyl and Latex Exam Glove Durability. The Journal of Testing and Evaluation 30(5):415-420; Korniewicz D. M., El-Masri M., Broyles J. M., et al., 2002 April. Performance of Latex and Nonlatex Medical Examination Gloves during Simulated Use. American Journal of Infection Control, 30(2):133-8; and Rego A., Roley L., 1999 October. In-Use Barrier Integrity of Gloves: Latex and Nitrile Superior to Vinyl. American Journal of Infection Control, 27(5):405-410. Given that polyvinyl chloride is inherently a much weaker material in terms of tensile strength and is likely to have pinholes in the membrane, polyvinyl chloride medical exam gloves require the use of a greater amount of material to achieve the same level of strength and integrity as a nitrile rubber medical exam glove. In view of these and other factors, consumers are beginning to seek an alternative to polyvinyl chloride gloves.
A need exists for an inexpensive, nitrile rubber glove that has good barrier properties at a cost that is less expensive than traditional nitrile rubber gloves or comparable to polyvinyl chloride gloves. Moreover, a need exists for an inexpensive nitrile rubber medical exam glove that can successfully provide the benefits of nitrile rubber materials while also providing pliability or softness like natural rubber latex without the conditions required for softening caused by stress relaxation. The present invention provides a simple solution to this need by means of a modified nitrile rubber-based synthetic polymer that exhibits not only good chemical resistance, but also stretch and silky tactile characteristics similar to natural rubber latex.